The immune system must decide whether to mount an immune response to a particular entity or whether to ignore the latter outcome as referred to as immunological tolerance. A great deal has been learned about tolerance over the past decade, but our understanding is still fragmentary, despite its importance for understanding autoimmune disease, managing acceptance of organ transplants, promoting cancer immunotherapy, and decreasing the complication of allosensitization by transfusion of platelets and other blood products. In the previous project period, we unexpectantly found that mice deficient in the Lyn tyrosine kinase have B lymphocytes that exhibit elevated responsiveness to antigenic stimulation. In vivo, these mice make high levels of autoantibodies directed at nuclear components such as double-stranded DNA and some of them develop kidney disease. Double mutant mice defective in Lyn and another Src-family kinase, Fyn were found to develop a much more severe autoimmune lupus-like kidney disease, with 50% of the animals dying by 7 months of age. We hypothesize that the defect in lyn makes B cells hyperresponsiveness and also defective in tolerance induction, resulting in production of antibodies directed at nuclear components released by apoptotic cells. We further hypothesize that the defect in fyn contributes to more rapid disease, possibly by making the kidneys more susceptible to damage resulting from immune complex deposition. These hypotheses will be tested by four Specific Aims. 1) We shall define the cellular basis of defects leading to autoimmune diseases in lyn-/-,fyn-/- mice. This will be done by bone marrow transplantation and by adoptive transfer of mature lymphocytes. 2) We shall define the effects of the Fyn- deficiency on B cell phenotype, signaling ability, and activation, both in the context of otherwise normal mice and in the context of the Lyn- deficiency. We hypothesize that in contrast to Lyn, Fyn is a positive element in antigen receptor signaling, so its loss will decrease B cell responsiveness to antigen. 3) We shall determine the effects of Lyn and Fyn deficiencies on tolerance to double-stranded DNA using Ig transgenic mice developed by Martin Weigert and coworkers. 4) We shall determine the effects of Lyn and Fyn deficiencies on allosensitization by platelet transfusions to test the hypothesis that genetic affecting lymphocyte responsiveness may cause individuals to be more or less likely to make an alloantibody response following transfusions.